In‐Channel Placement of Structure to Enhance Habitat Complexity and Connectivity for Stream‐Dwelling Salmonids

Abstract

Mechanisms responsible for observed responses of fish populations to manipulations of physical habitat are poorly understood. This chapter provides an overview of research conducted to investigate the response of salmonids to the placement of physical structure in streams and rivers and associated infrastructure (e.g. culverts) to restore degraded habitat complexity and connectivity. Although benefits of placement of physical structure are recognised, the response may be superficial, reflecting local increases in density rather than enhanced productivity and recruitment for the system as a whole. Inconclusive results and failures may be inconsistently reported, limiting the ability of managers to assess the effectiveness of alternative techniques. Biological response to physical habitat is complex and efficiency of techniques varies over a range of biotic and abiotic conditions raising concerns about their transferability to other regions or species. There is a need to develop robust audit protocols for all restoration projects undertaken against original objectives set, rather than report only perceived positive outcomes to the community of end-users. To address the lack of understanding of mechanisms that underpin the biological response to physical habitat, it is necessary to integrate the results of fine-resolution experimental studies with field observations of large-scale manipulation of structure.

Effects of boulder addition on European grayling (Thymallus thymallus) in a channelized river in Sweden

“…N€ aslund 1989;van Zyll De Jong et al 1997;Palm 2007). One biological process underlying this positive effect may be that increased structural heterogeneity increases the number of available territories for fish, allowing more trout to find protection and feeding positions (Palm 2007;Kemp 2010). Both adult and juvenile trout establish and defend feeding territories (Klemetsen et al 2003) and the amount of available territories may regulate a trout population in a stream (Elliott 1994;Milner et al 2003).…”

“…Efforts to rehabilitate channelized river habitats are ongoing both in Sweden and internationally (Roni et al 2002;Nilsson et al 2005;Roni 2005). By placing boulders and woody debris back into the stream, one seeks to increase structural heterogeneity and hence increase the amount of rearing and nursing habitats for stream-dwelling salmonids (Lepori et al 2005;Kemp 2010). The effect of such streambed restorations has overwhelmingly been evaluated based on the response of salmon or trout populations, and positive effects on these salmonids have been demonstrated in several studies, both in Sweden (N€ aslund 1989;Palm 2007) and globally (Roni et al 2002;Roni 2005).…”